A porous coal high in water content by percentage is low in calorific value. Thus, developments have been made about a method for yielding refined-coal from such a porous coal (see JP 07-233383 A). This production method is described herein. Initially, porous coal (raw coal) is pulverized into a particulate form. Thereafter, the particles are mixed with a heavy oil component and a solvent oil component to yield a raw material slurry. Next, the raw material slurry is preheated and then heated to advance the dehydration of the porous coal, and further the inside of pores in the porous coal is impregnated with a mixed oil to yield a dehydrated slurry. Thereafter, from the dehydrated slurry, a porous refined-coal and the mixed oil are separated, and then the porous refined-coal is dried (liquid-removed). The dried porous refined-coal is cooled and formed into a shape if desired. The mixed oil recovered at the time of the separation thereof from the dehydrated slurry or the drying is reused as a circulating oil to be again used for a raw material slurry.
In this refined-coal production method, the drying of the particulate porous refined-coal separated from the dehydrated slurry is usually attained by an indirect heat drying unit. As this indirect heat drying unit, the so-called steam tube drier is frequently used from the viewpoint of drying power and operability. This steam tube drier has a rotary cylinder located to be freely rotatable around an axis, plural heating tubes arranged inside the rotary cylinder to be parallel with the axis, and a carrier gas passing means for supplying a carrier gas from one out of two sides of the rotary cylinder to be discharged into the other side. As a method for drying a particulate matter, using such an indirect heat drying unit, suggested is a method of using two indirect heat drying units, supplying a carrier gas into first one of the indirect heat drying units to produce a co-current flow relative to the flow of the particulate matter, thereby drying the particulate matter, and next supplying the carrier gas into second one of the units to produce a counterflow against the flow of the particulate matter to dry the particulate matter further (see JP 2009-97783 A). The document states that such a use of two indirect heat drying units makes it possible to restrain the particulate matter from adhering onto the heating tubes, so as to yield a dried matter having a stable liquid content by percentage. Furthermore, in this drying method, for collecting dust in the carrier gas discharged from the first indirect heat drying unit, a first bug filter is used, and for collecting dust in the carrier gas discharged from the second indirect heat drying unit, a second bug filter is used. The respective dusts (fine particles) collected therein are mixed with a dried matter (particulate matter) discharged from the first indirect heat drying unit, and then the mixture is dried in the second indirect heat drying unit.
In this way, the collected dust is again dried to be made into a dried matter, whereby the resultant product (dried matter) is improved in yield. However, particularly, in the second indirect heat drying unit, for drying the particulate matter that has been dried to some degree, fine particles accompanying the carrier gas are increased. Consequently, the accumulation of the dust (the fine particles) into the second bug filter increases a loss in the pressure inside the bug filter, so that the balance between the pressures of the carrier gas is disturbed to hinder a stable driving of the processing, or to shorten the filter lifespan of the second bug filter. Moreover, the disturbance of carrier gas pressure balance causes the pressure inside the system to be partially changed to a negative pressure. Thus, the air is incorporated into the system to raise the concentration of oxygen, so that the dried matter may unfavorably ignite.
In the meantime, about the refined-coal yielded through the above-mentioned production method, dust is easily generated therefrom by the presence of the dried fine particles. Thus, for example, water is sprayed onto the resultant refined-coal. However, a large quantity of water is required. Moreover, the spray of the water causes an inconvenience of lowering the calorie value.
Patent Literature 1: JP 07-233383 A
Patent Literature 2: JP 2009-97783 A